Add console errors and usecounter when srcset candidates are dropped

Add console errors and usecounter when srcset candidates are dropped

Debugging markup is hard. 
I'm adding console logs here so that it'd be easier for authors to spot that they have invalid srcset descriptors,
and that their candidate resources are being dropped.

I also added a UseCounter, so we'd have a better idea on how often authors get their descriptors wrong.

BUG=422049
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=184069

[Yoav Weiss, 2014-10-21 06:09:34]

yoav@yoav.ws changed reviewers:
+ mkwst@chromium.org

[Mike West, 2014-10-21 06:40:54]

LGTM.

https://codereview.chromium.org/649183007/diff/40001/LayoutTests/fast/hidpi/i...
File
LayoutTests/fast/hidpi/image-srcset-density-and-width-descriptors-expected.txt
(right):

https://codereview.chromium.org/649183007/diff/40001/LayoutTests/fast/hidpi/i...
LayoutTests/fast/hidpi/image-srcset-density-and-width-descriptors-expected.txt:1:
CONSOLE ERROR: srcset has multiple 'x' descriptors or a mix of 'x' and 'w'/'h'
descriptors.
I'd suggest adding some context to these: "Failed to parse a 'srcset'
attribute's value: ...".

[Mike West, 2014-10-21 09:36:25]

SLGTM.

https://codereview.chromium.org/649183007/diff/60001/Source/core/html/parser/...
File Source/core/html/parser/HTMLSrcsetParser.cpp (right):

https://codereview.chromium.org/649183007/diff/60001/Source/core/html/parser/...
Source/core/html/parser/HTMLSrcsetParser.cpp:187: StringBuilder errorMessage;
StringBuilder is probably overkill here. '+' would be fine. But, you know,
whatever. You already wrote it, and it's marginally more efficient. :)

[Yoav Weiss, 2014-10-21 09:37:48]

The CQ bit was checked by yoav@yoav.ws

[commit-bot: I haz the power, 2014-10-21 09:39:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/649183007/60001

[commit-bot: I haz the power, 2014-10-21 10:47:41]

Committed patchset #4 (id:60001) as 184069